TABLE OF CONTENTs

5 Water Quality Impact 5-1

5.1 Introduction. 5-1

5.2 Environmental Legislation, Policies, Plans, Standards and Criteria. 5-1

5.3 Water Sensitive Receivers. 5-4

5.4 Description of the Environment and Baseline Conditions. 5-4

5.5 Identification of Potential Impacts. 5-7

5.6 Assessment Approach and Methodology. 5-7

5.7 Prediction and Evaluation of Environmental Impacts. 5-10

5.8 Cumulative Impacts from Concurrent Project 5-15

5.9 Mitigation of Environmental Impacts. 5-15

5.10 Evaluation of Residual Impacts. 5-19

5.11 Environmental Monitoring and Audit 5-19

5.12 Conclusion. 5-19

TABLES

Table 5.1 Summary of Water Quality Objectives for North Western WCZ

Table 5.2 Summary of Water Quality Statistics for the North Western WCZ in 2016

Table 5.3 Coastal Developments Incorporated in Water Quality Modelling Scenarios

Table 5.4 Depth Averaged Current Velocities at Assessment Points

Table 5.5 Comparison Results of Momentary Flow at Representative Cross-sections

Table 5.6 Comparison Results of Accumulated Flow at Representative Cross-sections

FIGURES

Figure 5.1 Indicative Locations of Representative Water Sensitive Receivers and Observation Points

Figure 5.2 Coastal Developments Incorporated in the Baseline and Proposed Scenarios

Figure 5.3 Locations of Indicator Points and Cross Sections

Figure 5.4 Locations of Assessment Points and Cross-Sections

APPENDICES

Appendix 5.1 Model Grid Layout and Properties

Appendix 5.2 Mathematical Expressions for Pile Frictions

Appendix 5.3 Model Validation

Appendix 5.4 Hydrodynamic Modelling Results

Appendix 5.5 Schematic Diagram of Cofferdam and Steel Pile Casing with Silt Curtain

5 Water Quality Impact

5.1 Introduction

5.1.1 This section presents the assessment on potential water quality impact arising from construction and operation of the Project, which has been conducted in accordance with the criteria and guidelines as stated in Annexes 6 and 14 of the Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM) as well as the requirements given in Clause 3.4.5 and Appendix C of the EIA Study Brief (No. ESB-302/2017).

5.1.2 The potential water quality impact arising from operational phase of the Project has been assessed with the use of the computational modelling approach. The proposed calibration and validation, the modelling parameters, model coverage area, and grid schematization for water quality model simulation, and cumulative impacts due to other projects, activities or pollution sources within a boundary have been agreed with the Director of EPD in accordance with Appendix C-1 of the EIA Study Brief (No. ESB-302/2017).

5.2 Environmental Legislation, Policies, Plans, Standards and Criteria

Technical Memorandum on Environmental Impact Assessment Ordinance (EIAO-TM)

5.2.1 The EIAO-TM was issued by EPD under Section 16 of the EIAO. Reference sections in the EIAO-TM provide the details of assessment criteria and guidelines that are relevant to the water quality assessment, including:

· Annex 6 - Criteria for Evaluating Water Pollution; and

· Annex 14 - Guidelines for Assessment of Water Pollution.

Water Quality Objectives

5.2.2 The Water Pollution Control Ordinance (WPCO) provides the major statutory framework for the protection and control of water quality in Hong Kong. According to the WPCO and its subsidiary legislation, Hong Kong waters are divided into ten Water Control Zones (WCZs). Corresponding statements of Water Quality Objectives (WQOs) are stipulated for different water regimes (marine waters, inland waters, bathing beaches subzones, secondary contact recreation subzones and fish culture subzones) in the WCZs based on their beneficial uses. The Project site is located within the North Western WCZ. The WQOs for the North Western WCZ is listed in Table 5.1. These WQOs were used as the water quality assessment criteria for the Project.

Table 5.1 Summary of Water Quality Objectives for North Western WCZ

Parameters	Objectives	Sub-Zone
Offensive Odour, Tints	Not to be present	Whole zone
Visible foam, oil scum, litter	Not to be present	Whole zone
Dissolved Oxygen (DO) within 2 m of the seabed	Not less than 2.0 mg/L for 90% of sampling occasions during the whole year	Marine waters
Depth-averaged DO	Not less than 4.0 mg/L	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones, Water Gathering Ground Subzones and other inland waters
	Not less than 4.0 mg/L for 90 % of the sampling occasions during the whole year	Marine waters
pH	To be in the range of 6.5 - 8.5, change due to human activity not to exceed 0.2	Marine waters excepting Bathing Beach Subzones
	To be in the range of 6.5 – 8.5	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
	To be in the range of 6.0 –9.0	Other inland waters
	To be in the range of 6.0 –9.0 for 95% samples collected during the whole year and waste discharges shall not cause the natural pH range to be extended by more than 0.5 units	Bathing Beach Subzones
Salinity	Change due to human activity not to exceed 10% of ambient	Whole zone
Temperature	Change due to human activity not to exceed 2 ^oC	Whole zone
Suspended solids (SS)	Waste discharge not to raise the ambient level by 30% caused, nor cause the accumulation of suspended solids which may adversely affect aquatic communities	Marine waters
	Not to cause the annual median to exceed 20 mg/L	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
	Not to cause the annual median to exceed 25 mg/L	Inland waters
Unionized Ammonia (UIA)	Annual mean not to exceed 0.021 mg/L as unionized form	Whole zone
Nutrients	Shall not cause excessive algal growth	Marine waters
Total Inorganic Nitrogen (TIN)	Annual mean depth-averaged inorganic nitrogen not to exceed 0.3 mg/L	Castle Peak Bay Subzone
	Annual mean depth-averaged inorganic nitrogen not to exceed 0.5 mg/L	Marine waters excepting Castle Peak Bay Subzone
Bacteria	Not exceed 610 per 100ml, calculated as the geometric mean of all samples collected in one calendar year	Secondary Contact Recreation Subzones
	Should be less than 1 per 100 ml, calculated as the running median of the most recent 5 consecutive samples taken between 7 and 21 days.	Tuen Mun (A) and Tuen Mun (B) Subzones and Water Gathering Ground Subzones
	Not exceed 1000 per 100 ml, calculated as the running median of the most recent 5 consecutive samples taken between 7 and 21 days	Tuen Mun (C) Subzone and other inland waters
	Not exceed 180 per 100 ml, calculated as the geometric mean of all samples collected from March to October inclusive. Samples should be taken at least 3 times in one calendar month at intervals of between 3 and 14 days.	Bathing Beach Subzones
Colour	Not to exceed 30 Hazen units	Tuen Mun (A) and Tuen Mun (B) Subzones and Water Gathering Ground Subzones
	Not to exceed 50 Hazen units	Tuen Mun (C) Subzone and other inland waters
5-Day Biochemical Oxygen Demand (BOD5)	Not to exceed 3 mg/L	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
	Not to exceed 5 mg/L	Inland waters
Chemical Oxygen Demand (COD)	Not to exceed 15 mg/L	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
	Not to exceed 30 mg/L	Inland waters
Toxins	Should not cause a risk to any beneficial uses of the aquatic environment	Whole zone
	Waste discharge shall not cause the toxins in water significant to produce toxic carcinogenic, mutagenic or teratogenic effects in humans, fish or any other aquatic organisms.	Whole zone
Phenol	Quantities shall not sufficient to produce a specific odour or more than 0.05 mg/L as C6 H5OH	Bathing Beach Subzones
Turbidity	Shall not reduce light transmission substantially from the normal level	Bathing Beach Subzones

Source: Statement of Water Quality Objectives (North Western Water Control Zone)

Technical Memorandum on Effluent Discharge Standard

5.2.3 Besides setting the WQOs, the WPCO controls effluent discharging into the WCZs through a licensing system. Guidance on the permissible effluent discharges based on the type of receiving waters (foul sewers, stormwater drains, inland and coastal waters) is provided in the “Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters” (TM-DSS), issued under Section 21 of the WPCO. The limits given in the TM-DSS cover the physical, chemical and microbial quality of effluents. Any discharge during the construction and operational stages should comply with the standards for effluent discharged into the foul sewers, inshore waters and marine waters of the North Western WCZ as stipulated in the TM-DSS.

Practice Note

5.2.4 A practice note for professional persons has been issued by the EPD to provide guidelines for handling and disposal of construction site discharges. The Practice Note for Professional Persons on Construction Site Drainage (ProPECC PN 1/94) "Construction Site Drainage" provides good practice guidelines for dealing with ten types of discharge from a construction site. These include surface runoff, groundwater, boring and drilling water, bentonite slurry, water for testing and sterilisation of water retaining structures and water pipes, wastewater from building construction, acid cleaning, etching and pickling wastewater, and wastewater from site facilities. Guidelines given in the ProPECC PN 1/94 should be followed as far as possible during construction to minimise the water quality impact due to construction site drainage.

5.2.5 The ProPECC PN 5/93 "Drainage Plans subject to Comments by Environmental Protection Department" provides guidelines and practices for handling, treatment and disposal of various effluent discharges to stormwater drains and foul sewers. The design of site drainage and disposal of various site effluents generated within the Project should follow the relevant guidelines and practices as given in the ProPECC PN 5/93.

5.3 Water Sensitive Receivers

5.3.1 Key marine Water Sensitive Receivers (WSRs) within the study area in North Western WCZ are identified with reference to Annex 14 of the EIAO-TM and their indicative locations are shown in Figure 5.1. The identified WSRs include.

· Cooling Water Intakes;

· Flushing Water Intakes;

· Bathing Beaches (both gazetted and non-gazetted);

· Typhoon Shelter;

· Marine Park;

· Corals;

· Artificial Reefs;

· Horseshoe Crab;

· Seagrass;

· Fish Spawning Ground;

· Site of Special Scientific Interest (SSSI);

· Proposed Marina at Tung Chung East Reclamation

· Mangrove Communities; and

· Committed / Potential Marine Park.

5.4 Description of the Environment and Baseline Conditions

5.4.1 The marine water quality monitoring data routinely collected by EPD were used to establish the baseline condition. A summary of marine water quality data collected in 2016 is presented in Table 5.2 for the monitoring stations in North Western WCZ (NM1, NM2, NM3, NM5, NM6 and NM8). Monitoring data for North Western Supplementary WCZ was not available from EPD's routine monitoring programme. Descriptions of the baseline conditions for individual WCZ provided in the subsequent sections are directly extracted from the EPD's report "Marine Water Quality in Hong Kong 2016".

5.4.2 In 2016, the North Western WCZ attained an overall WQO compliance rate of 72%. All stations in the WCZ fully complied with the UIA and DO objectives. Apart from NM1, all the other five stations in this WCZ did not meet the TIN objective. The relatively high levels of TIN (annual mean 0.44-0.78 mg/L) were likely attributed to the higher background level of Pearl River, and some local discharges and surface runoff from the Northwestern New Territories as well as North Lantau.

5.4.3 Sediment Sampling and Testing Plan (SSTP) has been conducted to establish the sediment quality within the study area. Details on the sediment quality can be referred to Section 6 and Appendix 6.3.

Table 5.2 Summary of Water Quality Statistics for the North Western WCZ in 2016

		Lantau Island (North)	Pearl Island	Pillar Point	Urmston Road	Chek Lap Kok		WPCO WQO (in marine waters)
						(North)	(West)
Parameter		NM1	NM2	NM3	NM5	NM6	NM8
Temperature (^oC)		23.3 (16.2 - 28.5)	23.6 (15.6 - 28.7)	23.7 (15.9 - 28.6)	23.7 (15.8 - 28.8)	24.0 (15.5 - 28.8)	24.0 (15.8 - 28.7)	Not more than 2 ^oC in daily temperature range
Salinity		28.6 (22.4 - 30.7)	26.0 (15.9 - 30.2)	26.2 (16.0 - 30.7)	24.9 (17.2 - 29.7)	22.5 (13.5 - 29.7)	24.2 (9.4 - 30.8)	Not to cause more than 10% change
Dissolved Oxygen (DO) (mg/L)	Depth average	5.6 (4.0 - 8.0)	5.8 (4.0 - 8.2)	5.6 (4.2 - 8.1)	5.7 (4.1 - 7.9)	6.0 (4.8 - 8.5)	6.1 (4.6 - 7.8)	Not less than 4 mg/L for 90% of the samples
	Bottom	5.2 (2.2 - 8.1)	5.5 (3.7 - 8.3)	5.4 (3.0 - 8.1)	5.3 (2.6 - 8.0)	5.8 (4.1 - 8.2)	5.8 (4.1 - 7.5)	Not less than 2 mg/L for 90% of the samples
Dissolved Oxygen (DO) (% Saturation)	Depth average	76 (57 - 98)	78 (60 - 100)	76 (62 - 99)	76 (56 - 96)	81 (64 - 103)	83 (65 - 99)	Not available
	Bottom	71 (31 - 99)	75 (53 - 102)	73 (42 - 99)	72 (37 - 98)	77 (57 - 99)	79 (56 - 99)	Not available
pH		7.8 (7.4 - 8.3)	7.9 (7.4 - 8.3)	7.8 (7.4 - 8.3)	7.8 (7.4 - 8.1)	7.8 (7.4 - 8.3)	7.9 (7.4 - 8.3)	6.5 - 8.5 (± 0.2 from natural range)
Suspended Solids (SS) (mg/L)		7.5 (1.6 - 24.3)	6.7 (1.5 - 18.7)	9.3 (1.9 - 25.0)	10.2 (2.4 - 20.3)	8.6 (2.5 - 23.1)	13.9 (1.9 - 31.5)	Not more than 30% increase
5-day Biochemical Oxygen Demand (BOD5) (mg/L)		0.8 (0.3 - 1.5)	0.7 (0.3 - 1.5)	0.7 (0.3 - 1.1)	0.7 (0.4 - 1.6)	1.1 (0.3 - 5.0)	0.7 (0.1 - 1.7)	Not available
Ammonia Nitrogen (NH3-N) (mg/L)		0.103 (0.033 - 0.197)	0.114 (0.053 - 0.273)	0.122 (0.054 - 0.283)	0.138 (0.054 - 0.303)	0.112 (0.030 - 0.347)	0.071 (0.024 - 0.227)	Not available
Unionised Ammonia (UIA) (mg/L)		0.003 (0.001 - 0.008)	0.003 (0.001 - 0.010)	0.004 (0.002 - 0.011)	0.003 (0.002 - 0.006)	0.003 (0.001 - 0.007)	0.002 (<0.001 - 0.004)	Not more than 0.021 mg/L for annual mean
Nitrite Nitrogen (NO2-N) (mg/L)		0.045 (0.016 - 0.086)	0.058 (0.019 - 0.096)	0.056 (0.019 - 0.107)	0.072 (0.024 - 0.115)	0.077 (0.027 - 0.143)	0.056 (0.019 - 0.094)	Not available
Nitrate Nitrogen (NO3-N) (mg/L)		0.296 (0.167 - 0.547)	0.434 (0.183 - 0.893)	0.397 (0.190 - 0.643)	0.516 (0.217 - 0.837)	0.591 (0.280 - 1.133)	0.486 (0.103 - 1.367)	Not available
Total Inorganic Nitrogen (TIN) (mg/L)		0.44 (0.31 - 0.63)	0.61 (0.34 - 1.04)	0.58 (0.34 - 0.80)	0.73 (0.39 - 1.15)	0.78 (0.41 - 1.31)	0.61 (0.20 - 1.54)	Not more than 0.5 mg/L for annual mean
Total Nitrogen (TN) (mg/L)		0.84 (0.56 - 1.10)	0.85 (0.42 - 1.22)	0.95 (0.54 - 1.35)	1.03 (0.57 - 1.57)	1.05 (0.60 - 1.47)	0.94 (0.45 - 1.72)	Not available
Orthophosphate Phosphorus (PO4) (mg/L)		0.025 (0.004 - 0.053)	0.029 (0.004 - 0.054)	0.030 (0.008 - 0.056)	0.033 (0.005 - 0.060)	0.031 (0.002 - 0.062)	0.021 (0.003 - 0.048)	Not available
Total Phosphorus (TP) (mg/L)		0.07 (0.04 - 0.12)	0.10 (0.04 - 0.14)	0.10 (0.05 - 0.16)	0.11 (0.06 - 0.17)	0.11 (0.05 - 0.16)	0.09 (0.03 - 0.13)	Not available
Chlorophyll-a (µg/L)		2.7 (0.7 - 12.4)	3.0 (0.5 - 13.3)	2.1 (0.7 - 7.1)	2.2 (0.5 - 6.7)	5.2 (0.7 - 24.0)	4.1 (1.0 - 18.0)	Not available
E. coli (cfu/100 mL)		27 (3 - 78)	35 (5 - 70)	54 (7 - 420)	210 (46 - 2000)	26 (2 - 120)	5 (<1 - 79)	Not available

Note:

1. Except as specified, data presented are depth-averaged values calculated by taking the means of three depths: surface, mid-depth, bottom.

2. Data presented are annual arithmetic means of depth-averaged results except for E.coli and faecal coliforms that are annual geometric means.

3. Data in brackets indicate the ranges.

5.5 Identification of Potential Impacts

Construction Phase

5.5.1 The major sources of water quality impacts during construction phase of the Project would potentially include the following:

· Construction of marine bridge piles for the Bonded Vehicular Bridge;

· Wastewater discharges from general construction activities;

· Drainage and construction site runoff from land-based construction;

· Sewage effluent produced by construction workforce; and

· Accidental spillage of chemicals.

Operational Phase

5.5.2 The key operational phase issues would be related to the change in hydraulic friction due to the formation of bridge piles of the Bonded Vehicular Bridge, which may result in change of flow regime in the sea channel between the HKIA and the HKBCF, and its adjacent waters in North Western WCZ.

5.5.3 Potential water quality impacts may also arise from the road surface runoff with suspended solids and sewage flow generated from the proposed toilets during operational phase. Proper drainage systems with silt traps should be installed, maintained and cleaned at regular intervals. All the sewage flow generated from the proposed toilets should be collected and conveyed to the existing HKBCF sewerage system and sewage treatment plant for treatment.

5.6 Assessment Approach and Methodology

Construction Phase

5.6.1 No open sea dredging will be involved for construction of the Bonded Vehicular Bridge. It is expected that the installation of steel pile casing would only cause minor displacement of marine sediment, which will quickly settle without significant increase in suspended solids. Other land-based construction works, including construction site runoff, wastewater from general construction activities and accidental spillage of chemicals, have been identified and qualitatively assessed in Section 5.7. Appropriate good practice measures such as the practices outlined in ProPECC PN 1/94 “Construction Site Drainage” would be recommended to minimise the potential water quality impacts during construction phase.

Operational Phase

5.6.2 Hydrodynamic modelling is required to evaluate the change in the hydrodynamic regime due to the Project. Two hydrodynamic modelling scenarios have been conducted to evaluate the change in the hydrodynamic regime due to the bridge piers from this Project as follow:

· Scenario 1 - Scenario without the Bonded Vehicular Bridge; and

· Scenario 2 - Scenario with the Bonded Vehicular Bridge.

Modelling Tools

5.6.3 The Delft3D suite of models will be utilised in this modelling exercise as the modelling platform with the Deflt3D-FLOW module used for hydrodynamic simulation.

5.6.4 Delft3D-FLOW is a 3-dimensional hydrodynamic simulation programme which calculates non-steady flow and transport phenomena that result from tidal and meteorological forcing on a curvilinear, boundary fitted grid.

Model Grid Layout and Properties

5.6.5 The development of the detailed model to be adopted under this study is based on the model setup of the regional Update model which was developed under the "Update on Cumulative Water Quality and Hydrological Effect of Coastal Developments and Upgrading of Assessment Tool'' (Agreement No. CE 42/97, hereafter "Update Study"). The Update Model developed under the Update Study covers the entire Hong Kong waters, the Pearl Estuary and the Dangan Channel to incorporate all major influences on hydrodynamic and water quality. The detailed model to be used for this study makes reference to the Update Model. The model grid was refined in the study area to give a better representation of the hydrodynamic condition. The areas covered by the detailed model include the North Western WCZ and the adjacent outer waters. Appendix 5.1 shows the grid layout of the detailed model at the study area.

5.6.6 The detailed model consists of 34,950 grid cells. The areas covered by the detailed model include the North Western, Western Buffer and Deep Bay Water Control Zones (WCZs) and the adjacent Mainland waters including the Pearl River Estuary. Grid size at the open waters is less than 400m in general. The smallest grid cells are located in the western waters which are less than 70m x 70m. A close up of the model grid at the western waters is shown in Appendix 5.1.

5.6.7 The grid quality of the detailed model is generally good except in some areas at or close to the land boundary. In view of the small flow velocity at the land boundary, numerical errors associated with the change of orthogonality should be small. Therefore, the closed grid cells at the coastlines have been adjusted to form a grid line that is parallel to land boundary (rather than keeping these closed grid cells orthogonal). Orthogonality at open grid cells has been checked to be adequate. The grid properties of detailed model grid including orthogonality, N-smoothness and M-smoothness are shown in Appendix 5.1.

Vertical Layers

5.6.8 The hydrodynamic model is 3-dimensional with a total of 10 vertical water layers. The thickness of each water layer is defined in the model as a percentage of the water depth where the total sum of all the vertical layers should be 100%. All the vertical layers have been assigned to have the same vertical contribution. Thus, each of the vertical layers contributes 10% of the total water depth.

Boundary Conditions

5.6.9 The detailed model is linked to or nested within the Update Model, which was constructed, calibrated and verified under the Update Study. Computations are first carried out using the Update Model to provide open boundary conditions to the detailed model. The Update Model covers the whole Hong Kong and the adjacent outer waters including the Pearl River Estuary and Dangan Channel. The influence on hydrodynamics and water quality in these outer regions would be fully incorporated into the detailed model.

Simulation Periods

5.6.10 For each modelling scenario, the hydrodynamic simulations have been performed for both dry and wet seasons, and the simulation period covered a 15-day full spring-neap cycle (excluding the spin-up period) for each of the dry and wet seasons. A spin-up period of 7 days was adopted for hydrodynamic simulation. Hence, the hydrodynamic model simulation period will consist of a spin-up period of 7 days plus an actual simulation period of 15 days (total 22 days). This spin-up period has been checked under the past EIA study to be sufficient for producing acceptable modelling results.

Wind

5.6.11 For the purpose of water quality assessment, a north-eastern wind with a belonging wind speed of 5 m/s will be used for the dry season computations. The wet season computations will apply a south western wind of 5 m/s. These conditions are the same as those used for the Update Study.

Roughness

5.6.12 The roughness varies over the model area. For areas with a depth larger than -10 mPD (Principle Datum) a Manning value of 0.026 was used. For depths in between -5 and -10 mPD a value of 0.023 was applied. The shallow areas was given a Manning value of 0.022. This roughness schematization originates from the Update model calibration carried out under the EPD Update Study.

Eddy Viscosity and Diffusivity

5.6.13 The horizontal eddy viscosity and diffusivity will be set to a uniform value of 1.0 m²/s as applied in the Update Model. The minimum value for the background vertical eddy viscosity and diffusivity will be set to 0.00005 m²/s, as applied in the Update Model.

Coastline Configuration

5.6.14 Table 5.3 below indicates the committed / on-going / planned coastal developments incorporated into the coastline configurations for hydrodynamic modelling under this modelling exercise. The layouts for specific projects are shown in Figure 5.2.

Table 5.3 Coastal Developments Incorporated in Water Quality Modelling Scenarios

Coastal Development	Information Source on Project Layout
TM-CLKL	EIA Report for “Tuen Mun - Chek Lap Kok Link” (EIAO Register No.: AEIAR-146/2009)
Hong Kong - Zhuhai - Macao Bridge (HZMB) Hong Kong Boundary (BCF)	EIA Report for “Hong Kong - Zhuhai - Macao Bridge Hong Kong Boundary Crossing Facilities” (EIAO Register No.: AEIAR-145/2009)
HZMB Hong Kong Link Road (HKLR)	EIA Report for “Hong Kong - Zhuhai - Macao Bridge Hong Kong Link Road” (EIAO Register No.: AEIAR-144/2009)
Tung Chung New Town Development Extension (TCNTDE)	EIA Report for “Tung Chung New Town Extension” (EIAO Register No. AEIAR-196/2016)
Expansion of Hong Kong International Airport into a Three-Runway System (HKIA3RS)	EIA Report for “Expansion of Hong Kong International Airport into a Three-Runway System” (EIAO Register No.: AEIAR-185/2014)
Contaminated Mid Pit (CMP) at South Brothers	EIA Report for “New Contaminated Mud Marine Disposal Facility at Airport East / East Sha Chau Area” (EIAO Register No.: AEIAR-082/2004) (Remark: The hydrodynamic effect of the capped CMP was incorporated into the hydrodynamic model. The final level after capping of the CMP was assumed in the model under the 2 modelling scenarios)
CMP at East Sha Chau

Model Bathymetry

5.6.15 The bathymetry schematization of the detailed model is based on the depth data from the Charts for Local Vessels 2013 produced by the Hydrographic Office, Marine Department of HKSAR. The hydrodynamic effect of the Contaminated Mid Pit (CMP) at East Sha Chau and The Brothers has also been incorporated and the final level at the CMP after capping was assumed in the modelling scenarios.

Pile Frictions

5.6.16 As the dimensions of the bridge piers of the Bonded Vehicular Bridge are much smaller than the grid size, the exact pier configurations cannot be adopted in the model simulation. Instead, only the overall influence of the piles on the flow will be taken into account. This overall influence has been modelled by a special feature of the Delft3D-FLOW model, namely porous plate. Porous plates represent transparent structures in the model and will be placed along the model gridline where momentum can still be exchanged across the plates. The porosity of the plates is controlled by a quadratic friction term in the momentum to simulate the energy losses due to the presence of the piles. The forces on the flow due to a vertical pile or series of piles have been used to determine the magnitude of the energy loss terms. The mathematical expressions for representation of piles friction is based on the Delft 3D-FLOW module developed by Delft Hydraulics as given in Appendix 5.2.

Model Validation

5.6.17 The performance of the detailed model has been checked against with Update Model results. The results of the actual simulation periods (with sufficient spin-up periods) for water level, depth averaged flow speed, depth averaged flow directions, salinity predicted by the two models have been compared at two indicator points within the modelled area. The results of momentary flows and accumulated flows have been compared at two selected cross sections to check for the consistency. Locations of the selected indicator points and cross sections are shown in Figure 5.3. Momentary flow represents the instantaneous flow rate at a specific time in m³/s whereas accumulated flow represents the total flow accumulated at a specific time in m³. The comparison plots are given in Appendix 5.3. The comparison plots indicated that the model results predicted by both models were in general consistent with each other which implied that the model settings of the detailed model as well as the nesting procedures were carried out correctly. The difference in accumulated flows was caused by the difference in grid resolutions between the two models. As the detailed model has relatively finer grid resolution, it should have a more accurate representation of the bathymetry and coastline in the western waters as compared to the Update Model and hence the deviation are considered to be reasonable.

Model Output

5.6.18 The modelling results including accumulated flow, momentary flow and depth averaged current velocities from the two hydrodynamic modelling scenarios have been compared in Section 5.7 in order to evaluate the hydrodynamic change in the study area.

5.7 Prediction and Evaluation of Environmental Impacts

Construction Phase

Construction of Marine Bridge Piles

5.7.1 The bridge deck section will be designed to allow for the use of precast concrete construction method. The deck will be formed from precast concrete sections which will be manufactured at a casting yard offsite and joined together at their final positions on-site. This approach will minimise the extent and duration of construction activities required on-site and hence the potential environmental impacts on nearby sensitive receivers during construction.

5.7.2 Construction of the viaducts will generally involve the use of in-situ bored piles foundations founded on bedrock or seabed. All piling equipment would be set up on a barge after the installation of silt curtain, then the pile construction would be through the placing of steel pile casing at the pier site in which the seawater trapped inside the casing. A funnel would be placed at the top of pile casing during excavation. This construction method of creating a confined environment for excavation could minimise the release of contaminant into the water column and hence reduce the risk of disturbance to the seabed and the adjacent marine environment. Mechanical Grab and Reverse Circulation Drill will be used for excavation of soil and rock socket respectively and then installing steel reinforcement fixing with permanent casing for concreting. No open sea dredging of seabed will be involved for the Bonded Vehicular Bridge construction. The marine viaduct pile cap above high-tide level will be installed through construction of a cofferdam, which consists of using permanent precast panel. The seawater trapped inside the cofferdam would be pumped out to generate a dry working environment throughout the construction process. The bridge piers will be then constructed by traditional means.

5.7.3 No open sea dredging will be involved for construction of the Bonded Vehicular Bridge. It is expected that the installation of steel pile casing would only cause minor displacement of marine sediment, which will quickly settle without significant increase in suspended solids. Sediment excavation will only be carried out in a confined dry working environment. As mentioned in Section 6, the excavated marine-based sediments will be loaded onto the barge and transported to the designated disposal sites allocated by Marine Fill Committee (MFC). No barging points or conveyor systems will be established in the Project area. Notwithstanding the above, accidental release of excavated sediment during transport to the disposal areas by barges which will increase the SS and contaminant level of receiving water and deteriorate water quality. Adoption of the guidelines and good site practices for handling and disposal of the excavated marine-based sediments as specified in Section 5.9.12 would minimise the potential impacts. Potential water quality impact due to release of suspended solids, contaminants and nutrients from sediment excavation is therefore not expected. No adverse water quality impact due to construction of marine bridge piles is anticipated.

General Construction Activities

5.7.4 Various types of construction activities may generate wastewater. These include general cleaning and polishing, wheel washing, dust suppression and utility installation. These types of wastewater would contain high concentrations of suspended solids (SS). Various construction works may also generate debris and rubbish such as packaging, construction materials and refuse. Uncontrolled discharge of site effluents, rubbish and refuse generated from the construction works could lead to deterioration in water quality.

5.7.5 Effluent discharged from temporary site facilities should be controlled to prevent direct discharge to the neighbouring marine waters and storm drains. Such effluent may include wastewater resulting from wheel washing of site vehicles at site entrances. Debris and rubbish such as packaging, construction materials and refuse generated from the construction activities should also be properly managed and controlled to avoid accidental release to the local storm system and marine waters. Adoption of the guidelines and good site practices for handling and disposal of construction discharges as specified in Sections 5.9.4 to 5.9.15 would minimise the potential impacts.

Drainage and Construction Site Runoff

5.7.6 Potential pollution sources of site runoff may include:

· Runoff and erosion of exposed bare soil and earth, drainage channel, earth working area and stockpiles;

· Release of any bentonite slurries, concrete washings and other grouting materials with construction runoff or storm water;

· Wash water from dust suppression sprays and wheel washing facilities; and

· Fuel, oil and lubricants from maintenance of construction vehicles and equipment.

5.7.7 During rainstorms, site runoff would wash away the soil particles on unpaved lands and areas with the topsoil exposed. The runoff is generally characterised by high concentrations of SS. Release of uncontrolled site runoff would increase the SS levels and turbidity in the nearby water environment. Site runoff may also wash away contaminated soil particles and therefore cause water pollution.

5.7.8 Wind blown dust would be generated from exposed soil surfaces in the works areas. It is possible that wind blown dust would fall directly onto the nearby water bodies when a strong wind occurs. Dispersion of dust within the works areas may increase the SS levels in surface runoff causing a potential impact to the nearby sensitive receivers.

5.7.9 Construction site runoff and drainage may cause local water quality impacts. Increase in SS arising from the construction site could block the drainage channels. High concentrations of suspended degradable organic material in marine water could lead to reduction in dissolved oxygen (DO) levels in the water column.

5.7.10 It is important that proper site practice and good site management (as specified in the ProPECC PN 1/94 "Construction Site Drainage") be followed to prevent runoff with high level of SS from entering the surrounding waters. With the implementation of appropriate measures to control runoff and drainage from the construction site, disturbance of water bodies would be avoided and deterioration in water quality would be minimal. Thus, unacceptable impacts on the water quality are not expected, provided that the relevant mitigation measures as specified in the ProPECC PN 1/94 "Construction Site Drainage" as described in Sections 5.9.4 to 5.9.15 are properly implemented.

Sewage Effluent from Construction Workforce

5.7.11 During the construction of the Project, the workforce on site will generate sewage effluents, which are characterised by high levels of BOD, ammonia and E.coli counts. Potential water quality impacts upon the local drainage and fresh water system may arise from these sewage effluents, if uncontrolled.

5.7.12 The construction sewage should be handled by interim sewage treatment facilities, such as portable chemical toilets. Appropriate numbers of portable toilets should be provided by a licensed contractor to serve the large number of construction workers over the construction site. Based on the Drainage Services Department (DSD) Sewerage Manual, the sewage production rate for construction workers is estimated at 0.35 m³ per worker per day. For every 100 construction workers working simultaneously at the construction site, about 35 m³ of sewage would be generated per day. Provided that sewage is not discharged directly into the storm drains or marine waters adjacent to the construction site, and temporary sanitary facilities are used and properly maintained (as given in Sections 5.9.16 to 5.9.17), it is unlikely that sewage generated from the site would have a significant water quality impact.

Accidental Spillage of Chemicals

5.7.13 A large variety of chemicals may be used during construction activities. These chemicals may include petroleum products, surplus adhesives, spent lubrication oil, grease and mineral oil, spent acid and alkaline solutions/solvent and other chemicals. The use of these chemicals and their storage as waste materials has the potential to create impacts on the water quality of adjacent marine waters or storm drains if spillage occurs. Waste oil may infiltrate into the surface soil layer, or runoff into local water courses, increasing hydrocarbon levels. The potential impacts could however be mitigated by practical mitigation measures and good site practices (as given in Sections 5.9.18 to 5.9.20).

Operational Phase

Change of Hydrodynamic Regime

5.7.14 The hydrodynamic modelling results for flow velocity vectors and current velocities are presented in Appendix 5.4. The indicative locations of the assessment points (namely P1, P2 and P3) are shown in Figure 5.4. A summary of depth averaged velocities within the whole simulation period are presented in Table 5.4.

Table 5.4 Depth Averaged Current Velocities at Assessment Points

Assessment Point (refer to Figure 5.4)	Depth Averaged Current Velocities (m/s)
	Dry Season		Wet Season
	Scenario 1	Scenario 2	Scenario 1	Scenario 2
P1	0.06 ( 0.02 – 0.17 )	0.06 ( 0.01 – 0.16 )	0.09 ( 0.02 – 0.25 )	0.09 ( 0.02 – 0.25 )
P2	0.04 ( 0.01 – 0.12 )	0.04 ( 0.01 – 0.12 )	0.08 ( 0.01 – 0.17 )	0.08 ( 0.01 – 0.18 )
P3	0.01 ( <0.01 – 0.06 )	0.01 ( <0.01 – 0.06 )	0.03 ( 0.01 – 0.11 )	0.03 ( <0.01 – 0.12 )

Notes:

1. Data presented are arithmetic means of depth-averaged results in dry and wet seasons.

2. Data in brackets indicate the ranges.

5.7.15 According to the hydrodynamic modelling results as shown in Table 5.4, there is no significant change in averaged current velocities at all assessment points under both modelling scenarios. There are slightly change in the prevailing velocities at all assessment points (at P1 in dry season, at P2 and P3 in wet season) but the change in the tidal speeds are only up to 0.01 m/s. As the predicted change in current velocity would be small and localised at the sea channel, significant change in flow regime is not anticipated.

5.7.16 The timeseries plots for momentary flow and accumulated flow across the three representative cross-sections (namely C1, C2 and C3 as indicated in Figure 5.4) under both modelling scenarios are also presented in Appendix 5.4. The momentary flow and accumulated flow across the three representative cross-sections are summarised in Table 5.5 and Table 5.6 respectively.

Table 5.5 Comparison Results of Momentary Flow at Representative Cross-sections

Cross-section (refer to Figure 5.4)	Seasons	Momentary Flow (m^3/s)
		Scenario 1	Scenario 2	Difference
				(m^3/s)	(%)
C1	Dry	-54 ~ 61	-54 ~ 61	<0	<1%
	Wet	-58 ~ 70	-58 ~ 69	<0 ~ 1	2%
C2	Dry	-37 ~ 41	-37 ~ 41	<0	<1%
	Wet	-39 ~ 47	-39 ~ 46	<0 ~ 1	2%
C3	Dry	-14 ~ 15	-14 ~ 15	<0	<1%
	Wet	-15 ~ 18	-15 ~ 17	<0 ~ 1	2%

Table 5.6 Comparison Results of Accumulated Flow at Representative Cross-sections

Cross-section (refer to Figure 5.4)	Seasons	Accumulated Flow (m^3)
		Scenario 1	Scenario 2	Difference
				(m^3)	(%)
C1	Dry	7.32 x10^5	7.32 x10^5	<0.01 x10^5	<1%
	Wet	7.76 x10^5	7.76 x10^5	<0.01 x10^5	<1%
C2	Dry	4.80 x10^5	4.80 x10^5	<0.01 x10^5	<1%
	Wet	5.10 x10^5	5.09 x10^5	0.01 x10^5	<1%
C3	Dry	1.80 x10^5	1.80 x10^5	<0.01 x10^5	<1%
	Wet	1.91 x10^5	1.91 x10^5	<0.01 x10^5	<1%

5.7.17 The predicted momentary flow and accumulated flow across the three representative cross-sections would decrease with the presence of the marine bridge piles of the Bonded Vehicular Bridge. However, the change in momentary flow and accumulated flow is considered to be small (difference only up to 1 m³/s (2%) and 1,000 m³ (<1%) for momentary flow and accumulated flow respectively). As the predicted change in momentary flow and accumulated flow would be small, significant change in flushing capacity is not anticipated. No adverse hydrodynamic impact would therefore be expected.

Runoff from Road Surfaces

5.7.18 Potential water quality impact may also arise from the road surface runoff discharge during operational phase. The surface runoff may contain small amount of suspended solids that may cause water quality impacts to the nearby receiving marine water. However, impacts upon water quality will be minimal provided that a proper drainage system will be provided to receive surface runoff to the existing drainage system at the planning and design stages.

5.7.19 According to the DSD “Stormwater Drainage Manual”, annual rainfall in Hong Kong is around 2,200 mm. However, the Update Study suggested that only rainfall events of sufficient intensity and volume would give rise to runoff and that runoff percentage is about 44% and 82% for dry and wet season, respectively. Therefore, only 1,386 mm of 2,200 mm annual rainfall would be considered as effective rainfall that would generate runoff (i.e. 1,386 mm = 2,200 mm × (82%+44%)/2).

5.7.20 Additional surface runoff would be generated from the Project due to construction of bridge deck section above the sea channel between the HKIA and the HKBCF. The additional area is about 4,034 m². Making reference to the DSD “Stormwater Drainage Manual”, about 0.9 as the runoff coefficient for paved areas is assumed. The average daily runoff generated from the construction area is estimated to be less than 14 m³/day (= 0.9 × 1,386 mm/year × 4,034 m²).

5.7.21 It is also anticipated that with proper implementation of recommended mitigation measures and best management practices described in Sections 5.9.22, adverse impact associated with the discharge of runoff is not anticipated.

Sewage Effluent from the Proposed Toilets

5.7.22 According to latest sewerage review, the average dry weather flow (ADWF) generated from the proposed toilets is estimated to be 3.96 m³/day. All the sewage flow generated from the proposed toilets would be collected and conveyed to the existing sewerage system on HKBCF Island which conveys sewage towards the HKBCF sewage treatment plant. The downstream sewerage system and sewage treatment plant at the HKBCF have sufficient capacity to treat the additional sewage flow generated from the Project. No adverse water quality impacts would therefore be anticipated.

5.8 Cumulative Impacts from Concurrent Project

5.8.1 The construction and operation of the Project potentially overlap with the construction and operation of the Intermodal Transfer Terminal (ITT) and other nearby concurrent projects as identified in Table 5.3. However, with incorporation of the recommended mitigation measures during the construction and operational phases of this Project, no adverse cumulative water quality impacts would be expected.

5.9 Mitigation of Environmental Impacts

Construction Phase

Construction of Marine Bridge Piles

5.9.1 Major control measures relevant to this Project as listed in following sections (i.e. Sections 5.9.1 to 5.9.3) should be included in relevant contract documents. Steel pile casing and watertight cofferdam should be installed at the pier site and seawater trapped inside the casing and cofferdam should be pumped out to generate a dry working environment prior to carrying out sediment excavation.

5.9.2 During dewatering of the cofferdam, appropriate desilting or sedimentation device should be provided on site for treatment before discharge. The Contractor should ensure discharge water from the sedimentation tank meeting the WPCO / TM-DSS requirements before discharge.

5.9.3 To minimise any adverse water quality impact during the excavation of sediment, a funnel should be placed at the top of pile casing during excavation and silt curtains should be deployed to completely enclose the cofferdam and steel pile casing. Silt curtains should be deployed prior to setting up piling equipment on barge, installation of cofferdam and steel pile casing. Silt curtains should only be removed after completion of pile caps and piers. The Contractor should be responsible for the design, installation and maintenance of the silt curtain to minimise the impacts on water quality. The design and specification of the silt curtains should be submitted by the Contractor to the Project Manager of AAHK for approval. The marine bridge piers should not be constructed at the same time to avoid adverse hydrodynamic impact due to flow blockage increase during the interim construction stages. All vessels should be sized such that adequate clearance is maintained between vessels and the sea bed at all states of the tide to ensure that undue turbidity is not generated by turbulence from vessel movement or propeller wash. The schematic diagram of cofferdam and steel pile casing with silt curtain is indicatively shown in Appendix 5.5.

Construction Site Runoff and General Construction Activities

5.9.4 The site practices outlined in ProPECC PN 1/94 "Construction Site Drainage" should be followed as far as practicable to minimise surface run-off and the chance of erosion. The following measures are recommended to protect water quality and sensitive uses of the coastal area, and when properly implemented should be sufficient to adequately control site discharges so as to avoid water quality impact.

5.9.5 Surface run-off from construction sites should be discharged into storm drains via adequately designed sand/silt removal facilities such as sand traps, silt traps and sedimentation basins. Channels or earth bunds or sand bag barriers should be provided on site to properly direct stormwater to such silt removal facilities. Perimeter channels at site boundaries should be provided on site boundaries where necessary to intercept storm run-off from outside the site so that it will not wash across the site. Catchpits and perimeter channels should be constructed in advance of site formation works and earthworks.

5.9.6 Silt removal facilities, channels and manholes should be maintained and the deposited silt and grit should be removed regularly, at the onset of and after each rainstorm to prevent local flooding. Before disposal at the public fill reception facilities, the deposited silt and grit should be solicited in such a way that it can be contained and delivered by dump truck instead of tanker truck. Any practical options for the diversion and re-alignment of drainage should comply with both engineering and environmental requirements in order to provide adequate hydraulic capacity of all drains.

5.9.7 Construction works should be programmed to minimise soil excavation works in rainy seasons (April to September). If excavation in soil cannot be avoided in these months or at any time of year when rainstorms are likely, for the purpose of preventing soil erosion, temporary exposed slope surfaces should be covered e.g. by tarpaulin, and temporary access roads should be protected by crushed stone or gravel, as excavation proceeds. Intercepting channels should be provided (e.g. along the crest / edge of excavation) to prevent storm runoff from washing across exposed soil surfaces. Arrangements should always be in place in such a way that adequate surface protection measures can be safely carried out well before the arrival of a rainstorm.

5.9.8 Earthworks final surfaces should be well compacted and the subsequent permanent work or surface protection should be carried out immediately after the final surfaces are formed to prevent erosion caused by rainstorms. Appropriate drainage like intercepting channels should be provided where necessary.

5.9.9 Measures should be taken to minimise the ingress of rainwater into trenches. If excavation of trenches in wet seasons is necessary, they should be dug and backfilled in short sections. Rainwater pumped out from trenches or foundation excavations should be discharged into storm drains via silt removal facilities.

5.9.10 Manholes (including newly constructed ones) should always be adequately covered and temporarily sealed so as to prevent silt, construction materials or debris from getting into the drainage system, and to prevent storm run-off from getting into foul sewers. Discharge of surface run-off into foul sewers must always be prevented in order not to unduly overload the foul sewerage system.

5.9.11 If bentonite slurries are required for any construction works, they should be reconditioned and reused wherever practicable to minimise the disposal volume of used bentonite slurries. Temporary enclosed storage locations should be provided on-site for any unused bentonite that needs to be transported away after the related construction activities are completed. Requirements as stipulated in ProPECC Note PN 1/94 should be closely followed when handling and disposing bentonite slurries.

5.9.12 Good site practices should be adopted to remove rubbish and litter from construction sites so as to prevent the rubbish and litter from spreading from the site area. It is recommended to clean the construction sites on a regular basis. Also, as discussed in Sections 6.5.22 and 6.5.23, the following mitigation measures related to the transportation of the sediment should be implemented to minimise the potential water quality impact:

· Loading of the excavated marine-based sediment to the barge shall be controlled to avoid splashing and overflowing of the sediment slurry to the surrounding water;

· The barge transporting the excavated marine-based sediment to the designated disposal sites shall be equipped with tight fitting seals to prevent leakage and shall not be filled to a level that would cause overflow of materials or laden water during loading or transportation; and

· Monitoring of the barge loading shall be conducted to ensure that loss of material does not take place during transportation. Transport barges or vessels shall be equipped with automatic self-monitoring devices as specified by the Director of Environmental Protection (DEP).

Boring and Drilling Water

5.9.13 Water used in ground boring and drilling for site investigation or rock/soil anchoring should as far as practicable be re-circulated after sedimentation. When there is a need for final disposal, the wastewater should be discharged into storm drains via silt removal facilities.

Wheel Washing Water

5.9.14 All vehicles and plant should be cleaned before they leave a construction site to minimise the deposition of earth, mud, debris on roads. A wheel washing bay should be provided at every site exit if practicable and wash-water should have sand and silt settled out or removed before discharging into storm drains. The section of construction road between the wheel washing bay and the public road should be paved with backfall to reduce vehicle tracking of soil and to prevent site run-off from entering public road drains.

Site Effluent

5.9.15 There is a need to apply to EPD for a discharge licence for discharge of effluent from the construction site under the WPCO. The discharge quality must meet the requirements specified in the discharge licence. All the runoff and wastewater generated from the works areas should be treated so that it satisfies all the standards listed in the TM-DSS. The beneficial uses of the treated effluent for other on-site activities such as dust suppression, wheel washing and general cleaning etc., can minimise water consumption and reduce the effluent discharge volume. If monitoring of the treated effluent quality from the works areas is required during the construction phase of the Project, the monitoring should be carried out in accordance with the relevant WPCO license.

Sewage Effluent from Construction Workforce

5.9.16 No discharge of sewage to the storm water system and marine water will be allowed. Sufficient chemical toilets should be provided in the works areas to handle the sewage generated from the construction workforce. A licensed waste collector should be deployed to clean the chemical toilets on a regular basis.

5.9.17 Notices should be posted at conspicuous locations to remind the workers not to discharge any sewage or wastewater into the surrounding environment. Regular environmental audit of the construction site will provide an effective control of any malpractices and can encourage continual improvement of environmental performance on site. It is anticipated that sewage generation during the construction phase of the project would not cause water pollution problem after undertaking all required measures.

Accidental Spillage of Chemicals

5.9.18 Contractor must register as a chemical waste producer if chemical wastes would be produced from the construction activities. The Waste Disposal Ordinance (Cap 354) and its subsidiary regulations in particular the Waste Disposal (Chemical Waste) (General) Regulation, should be observed and complied with for control of chemical wastes.

5.9.19 Any service shop and maintenance facilities should be located on hard standings within a bonded area, and sumps and oil interceptors should be provided. Maintenance of vehicles and equipment involving activities with potential for leakage and spillage should only be undertaken within the areas appropriately equipped to control these discharges.

5.9.20 Disposal of chemical wastes should be carried out in compliance with the Waste Disposal Ordinance. The Code of Practice on the Packaging, Labelling and Storage of Chemical Wastes published under the Waste Disposal Ordinance details the requirements to deal with chemical wastes. General requirements are given as follows:

· Suitable containers should be used to hold the chemical wastes to avoid leakage or spillage during storage, handling and transport;

· Chemical waste containers should be suitably labelled, to notify and warn the personnel who are handling the wastes, to avoid accidents; and

· Storage area should be selected at a safe location on site and adequate space should be allocated to the storage area.

Operational Phase

Change of Hydrodynamic Regime

5.9.21 No adverse hydrodynamic impact would be expected during the operational phase and hence no mitigation measures are considered necessary.

Runoff from Road Surfaces

5.9.22 For the operation of road works, a surface water drainage system should be provided to collect the road runoff to the existing drainage system. The road drainage should be provided with adequately designed silt trap, as necessary. The design of the operational phase mitigation measures for the road works shall take into account the guidelines published in ProPECC PN 5/93 “Drainage Plans subject to Comment by the EPD”.

Best Storm Water Management Practices and Storm Water Pollution Control Plan

5.9.23 Mitigation measures including Best Management Practices (BMPs) to reduce storm water pollution arising from the Project are as follows:

Design Measures

5.9.24 Exposed surface shall be avoided within the roads to minimise soil erosion. The roads shall be hard paved.

5.9.25 The drainage system should be designed to avoid flooding.

Devices and Facilities

5.9.26 Screening facilities such as standard gully grating and trash grille, with spacing which is capable of screening large substances such as rubbish should be provided at the inlet of drainage system.

5.9.27 Road gullies with standard design and silt traps should be provided to remove particles present in stormwater runoff, where appropriate.

Administrative Measures

5.9.28 Good management measures such as regular cleaning and sweeping of road surface/ open areas are suggested. The road surface/ open area cleaning should also be carried out prior to occurrence rainstorm.

5.9.29 Manholes, as well as stormwater gullies, ditches provided at the Project site should be regularly inspected and cleaned (e.g. monthly). Additional inspection and cleansing should be carried out before forecast heavy rainfall.

Sewage Effluent from the Proposed Toilets

5.9.30 All the sewage flow generated from the proposed toilets should be properly collected and conveyed to the existing sewerage system on HKBCF Island. No direct discharge of sewage effluent into the marine water will be allowed.

5.10 Evaluation of Residual Impacts

Construction Phase

5.10.1 The construction phase water quality impact would generally be temporary and localised during construction. Therefore, no unacceptable residual water quality impact is anticipated during the construction of the Project, provided that all of the recommended mitigation measures as stated in Sections 5.9.1 to 5.9.20 are implemented and all construction site discharges comply with the TM-DSS standards.

Operational Phase

5.10.2 As presented in Sections 5.7.14 to 5.7.18, adverse hydrodynamic and water quality impacts associated with the operation of the Project are not anticipated. Thus, there will be no adverse residual impact associated with the operation of the Project.

5.11 Environmental Monitoring and Audit

Construction Phase

5.11.1 No adverse water quality impact was expected during the construction phase of the Project. Appropriate mitigation measures are recommended in Sections 5.9.1 to 5.9.20 to minimise potential water quality impacts. Water quality monitoring and audit is recommended during construction phase to ensure that all the recommended mitigation measures are properly implemented. Details of the water quality monitoring and audit programme and the Event and Action Plan are provided in the stand-alone EM&A Manual.

Operational Phase

5.11.2 No adverse hydrodynamic and water quality impacts would be expected from the Project. No monitoring programme specific for operational phase of the Project would be required.

5.12 Conclusion

Construction Phase

5.12.1 No open sea dredging will be involved for construction of the Bonded Vehicular Bridge. Sediment excavation will only be carried out in a confined dry working environment. With implementation of proposed mitigation measures as specified in Sections 5.9.1 to 5.9.3, no adverse water quality impact due to marine construction of the Bonded Vehicular Bridge would be expected. A water quality monitoring and audit programme will be implemented to ensure the effectiveness of the proposed water quality mitigation measures.

5.12.2 The potential water quality impacts from the land-based construction works are associated with the general construction activities, construction site run-off, accidental spillage, and sewage effluent from construction workforce. The site practices as outlined in the ProPECCPN 1/94 "Construction Site Drainage" is recommended to minimise the potential water quality impacts from the construction activities. Proper site management and good site practices are also recommended to ensure that construction wastes and other construction-related materials would not enter the nearby marine water. Sewage effluent arising from the construction workforce would be handled through provision of portable toilets. Water quality monitoring and regular site inspection will be implemented for the construction works to ensure that the recommended mitigation measures are properly implemented.

5.12.3 With the implementation of the above recommended mitigation measures, the land-based construction works for the Project would not result in adverse water quality impacts.

Operational Phase

5.12.4 Potential hydrodynamic impact due to the presence of the marine bridge piles of the Bonded Vehicular Bridge has been identified and assessed under this study with the use of computational modelling approach. The model results showed that the change in current velocity would be small and localised at the sea channel between HKIA and HKBCF Island. No significant change in flow regime at the sea channel is anticipated. The predicted change in momentary flow and accumulated flow would also be minor, significant change in flushing capacity is not anticipated. No adverse hydrodynamic impact would therefore be expected.